1. Field of the Invention
The present invention relates to testing of pixels of a flat panel display. In particular, embodiments of the invention relate to the testing of pixels by directing an electron beam onto a non-uniform electrode area.
2. Description of the Related Art
In years past, a common display for computers and other electronic products has been the cathode ray tube, or CRT. The CRT served as the standard display for personal computers (PC's) and televisions during the last half of the twentieth century. The CRT generally operates on a curved glass panel to form a display.
Recently, active matrix liquid crystal displays, or LCD's, have been commercially developed. The LCD has several advantages over the CRT, including higher picture quality, lighter weight, lower voltage requirements, and low power consumption. LCD displays are beneficial for flat panel displays, and have been commercialized of late in many portable electronic devices such as calculator screens, personal digital assistant (PDA) screens, portable computer (notebook) screens, mobile telephone displays, and small computer and television screens. In addition, larger LCD displays are now being employed in flat-screen televisions for the consumer market.
One type of active matrix LCD comprises liquid crystal material sandwiched between a TFT-array substrate and a color filter substrate. The TFT-array substrate comprises an array of thin film transistors (TFT's) each coupled to a pixel electrode. The color filter substrate comprises different color filter portions and a common electrode. When a certain voltage is applied to a pixel electrode, an electric field is created between the pixel electrode and the common electrode, orienting the liquid crystal material to allow light to pass therethrough for that particular pixel.
FIG. 1 is a schematic diagram showing one pixel of a liquid crystal display 100 comprising a uniform pixel electrode. This diagram is taken from the web site of Fujitsu, found currently at http://www.fme.fujitsu.com/products/displays/lcdvatech.html. Liquid crystal material 120 is sandwiched between the TFT-array substrate 110′ and the color filter substrate 110″. Since the TFT-array substrate 110′ comprises a uniform pixel electrode 112′, the molecules of the liquid crystal orient in a single direction when a certain voltage is applied to the pixel electrode. The light intensity of the display 100 is dependent on the view direction in reference to the liquid crystal orientation. Thus, the TFT-LCD having a uniform pixel electrode has a drawback in that the effective viewing angle is narrow.
FIG. 2 is a schematic diagram showing one embodiment of one pixel of a liquid crystal display 200 comprising a non-uniform electrode. This diagram is taken from the web site of Fujitsu, found currently at http://www.fme.fujitsu.com/products/displays/lcdvatech.html. Liquid crystal material 220A and 220B is sandwiched between the TFT-array substrate 210′ and the color filter substrate 210″. The TFT-array substrate 210′ comprises a non-uniform electrode 212′. The non-uniform electrode comprises dielectric lines formed over a conductive portion. The dielectric lines cause the liquid crystal material to orient in multiple directions. As a consequence, the display 200 seems bright when viewed at different angles by the viewer. This type of display having a non-uniform electrode comprising dielectric lines formed over a conductive portion is referred to as a multi-domain vertical alignment (MVA) display.
FIG. 3 and FIG. 4 are schematic diagrams of one example of a MVA display in which the liquid crystal can be oriented in four directions, designated as domains A, B, C, and D. In FIG. 4, six pixels can be seen having dielectric lines 218 formed over conductive portions 220B, G, and R. The dielectric lines 218 are used to divide and align the liquid crystal in a pixel into a plurality of alignment orientations.
Another type of display having a non-uniform electrode is referred to as an In Plane Switching (IPS) display. The display comprises a pair of electrodes formed over the TFT-array substrate. In one arrangement, the IPS display employs a pair of electrodes shaped as interlocking fingers. The liquid crystal molecules remain parallel to the substrates. As a consequence, the viewing angle of the display is increased.
As sizes increase for MVA-type and IPS-type displays, manufacturers must add more pixels and transistors to the substrate. Those of ordinary skill in the art appreciate that even moderately-sized color displays may employ transistors that number in the millions. If there is a problem with any of the transistors, it creates a defective pixel on the display. As the number of transistors increases, the likelihood that a bad transistor might be created within a display also increases. Therefore, manufacturers of large LCD's will test all or a percentage of pixels in a display as part of quality control.
Electron beam testing (EBT) can be used to monitor and troubleshoot defects during the manufacturing process. In a typical EBT process, TFT response within the pixels is monitored to provide defect information. For example, in EBT testing, certain voltages are applied to the thin film transistors, and the electron beam is directed to the individual pixel electrodes under investigation. Secondary electrons emitted from the pixel electrode area are sensed to determine the TFT voltages.
During testing, an electron beam is positioned over each pixel electrode of the TFT array, one after the other. To accomplish this movement, a substrate is first positioned on a table below an electron beam column. A substrate area (sometimes referred to as a sub-display area) may be moved under the electron beam column. Once a sub-display area of a substrate area is under the beam column, the beam is moved sequentially over each pixel electrode within the substrate area. After this area has been tested, the table is moved for testing of the next area. In some newer systems, two to four beams are utilized in parallel to simultaneously test separate substrate areas.
Electron beam testing of displays comprising non-uniform electrodes is problematic due to the conductive portions and dielectric portions of the non-uniform electrode. As a consequence, an improved method of testing displays comprising non-uniform electrodes is needed.